Three-year-old Preston Orlando shot himself the afternoon of July 5 with a Glock 30 that belonged to his father, Brandon Orlando.

The district attorney’s office announced that it would not charge Orlando with criminal storage of a firearm for the incident, which it called a “horrible irreversible mistake,” because there was no evidence that he had been grossly negligent.

Orlando told investigators he had returned to the family’s Gilroy home “exhausted” the morning of July 5 after working the overnight shift as a San Jose police officer, district attorney’s officials said.

He had left a loaded Glock, his backup service weapon, in a bedside nightstand drawer before falling asleep. Orlando’s wife and two children were out at the time, and he did not expect them to return home before he had to leave for his second job, according to the district attorney’s office.

But when Orlando’s second job was canceled for the day and his wife and children came home with Orlando’s nieces and nephews, Orlando forgot about leaving the gun next to his bed, district attorney’s officials said.

When Orlando went downstairs to do laundry with his wife, he heard a bang and went upstairs to find his son Preston with a self-inflicted gunshot wound above his right eye. Preston was pronounced dead shortly after.

In its decision not to bring charges against Orlando, the district attorney’s office also considered the impact of the boy’s death on the father. District attorney’s officials described him as a “devastated” man who is severely traumatized and has flashbacks and trouble sleeping.

“For Officer Orlando, this was a devastating mistake that can never be corrected,” District Attorney Jeff Rosen said in a statement. “For the rest of us, it is a sad, cautionary tale about the paramount importance of gun safety.”

Rosen reminded the public to keep weapons out of the reach of children.

(Copyright 2012 by CBS San Francisco and Bay City News Service. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)